Myofibrillar structure, with the emphasis on arrangement, number and behaviour of crossbridges, will be explored using X-ray diffraction, electron microscopy, immuno-histochemistry and quantitative mass determination by electron microscopy. Improvement of glycerol extraction procedures and of EM specimen preparation procedures will be monitored by X-ray diffraction to develop methods that best preserve the crossbridge lattice for investigation in thin sections. Optical diffraction and computerized image processing will be used to assist interpretation of EM images. Glycerinated muscle from frog, rabbit and insect, as well as live frog muscle will be used. The rigor crossbridge lattice will be used to quantify the proportion of myosin heads and actin subunits engaged, and to define whether slow filament sliding is possible in rigor. ATP analogues and modification of fibrillar proteins will be used to study and arrest the crossbridge cycle, to explore mechanochemical coupling. The changes in the vertebrate Z band lattice will be explored to see whether they can indicate twist on actin filaments during contraction. Comparative studies of variations in the crossbridge lattice of different mammalian muscles and of various insect flight muscles will be carried on after other methods and problems have made progress. Mechanical and computer models of the myofilament-crossbridge lattice will be prepared to summarize and assist interpretation. BIBLIOGRAPHIC REFERENCE: Reedy, M.K. (1976) A Band Periods in Vertebrate Muscle at Rest and in Rigor. J. Cell Biol. 70, 340a.